The Network Layer

1
The Network Layer
2
Purpose of Network layer

• Given a packet, send it across the network to
  destination
• 2 key issues
• Portability
• connect different technologies
• Scalability
• To the Internet scale

3
What does it involve?

• Two important functions
• routing determine path from source to dest.
• forwarding move packets from routers input to
  output

T3
T1 T3
Sts-1
T1
4
Network service model

• Q What service model for channel transporting
  packets from sender to receiver?
• guaranteed bandwidth?
• preservation of inter-packet timing (no jitter)?
• loss-free delivery?
• in-order delivery?
• congestion feedback to sender?

The most important abstraction provided by
network layer
?
?
virtual circuit or datagram?
?
service abstraction
Which things can be faked at the transport
layer?
5
Two connection models

• Connectionless (or datagram)
• each packet contains enough information that
  routers can decide how to get it to its final
  destination
• Connection-oriented (or virtual circuit)
• first set up a connection between two nodes
• label it (called a virtual circuit identifier
  (VCI))
• all packets carry label

1
A
6
Virtual circuits signaling protocols

• used to setup, maintain teardown VC
• setup gives opportunity to reserve resources
• used in ATM, frame-relay, X.25
• not used in todays Internet

6. Receive data
5. Data flow begins
4. Call connected
3. Accept call
1. Initiate call
2. incoming call
7
Virtual circuit switching

• Forming a circuit
• send a connection request from A to B. Contains
  VCI address of B
• rule VCI must be unique on the link its used on
• switch creates an entry mapping input messages
  with VCI to output port
• switch picks a new VCI unique between it and next
  switch

8
Virtual circuit forwarding

• For each VCI switch has a table which maps input
  link to output link and gives the new VCI to use
• if as messages come into switch 1 on link 2 and
  go out on link 3 then the table will be

(Input link,VCI) (output link, new VCI) (1,
2) (?, ?) (1, 5) (?, ?)
Switch 1
2
Switch 2
1
5
2
1
Switch 3
2
1
9
Virtual Circuits Discussion

• Plusses easy to associate resources with VC
• Easy to provide QoS guarantees (bandwidth, delay)
• Very little state in packet
• Minuses
• Not good in case of crashes
• Requires explicit connect and teardown phases
• What if teardown does not get to all routers?
• What if one switch crashes?
• Will have to teardown and rebuild route

10
Datagram networks

• no call setup at network layer
• routers no state about end-to-end connections
• no network-level concept of connection
• packets typically routed using destination host
  ID
• packets between same source-dest pair may take
  different paths
• Best effort data corruption, packet drops, route
  loops

1. Send data
2. Receive data
11
Datagrams Forwarding

• How does packet get to the destination?
• switch creates a forwarding table, mapping
  destinations to output port (ignores input ports)
• when a packet with a destination address in the
  table arrives, it pushes it out on the
  appropriate output port
• when a packet with a destination address not in
  the table arrives, it must find out more routing
  information (next problem)

12
Datagrams

• Plusses
• No round trip connection setup time
• No explicit route teardown
• No resource reservation ? each flow could get max
  bandwidth
• Easily handles switch failures routes around it
• Minuses
• Difficult to provide resource guarantees
• Higher per packet overhead
• Internet uses datagrams IP (Internet Protocol)

13
Datagrams Forwarding

• How to build forwarding tables?
• Manually enter it
• What if nodes crashed
• What about scale?
• The graph-theoretic routing problem
• Given a graph, with vertices (switches), edges
  (links), and edge costs (cost of sending on that
  link)
• Find the least cost path between any two nodes
• Path cost ? (cost of edges in path)

14
Simple Routing Algorithm

• Choose a central node
• All nodes send their (nbr, cost) information to
  this node
• Central node uses info to learn entire topology
  of the network
• It then computes shortest paths between all pairs
  of nodes
• Using All Pair Shortest Path Algorithm
• Sends the new matrix to every node
• Nice, simple, elegant!
• What is the problem?
• Scalability centralization hurts scalability
• Central node is crushed with traffic

15
Link State Routing

• Basic idea
• Every node propagates its (nbr, cost) information
• This information at all nodes is enough to
  construct topology
• Can use a graph algorithm to find the shortest
  routes
• Mechanisms required
• Reliable flooding of link information
• Method to calculate shortest route (Dijkstras
  algorithm)
• Example link state update packet
• node id, (nbr, cost) list, seq. no., ttl
• Seq. no. to identify latest updates, ttl
  specifies when to stop msg.

16
Reliable flooding

• receive(pkt)
• If already have a copy of LSP from pkt.ID
• if pkts sequence number lt copys
• discard pkt
• else
• decrement pkt.TTL
• replace copy with pkt
• forward pkt to all links besides the
• one that we received it on
• done every 10 minutes or so
• gen_LSP()
• increment nodes sequence by one
• recompute cost vector
• send created LSP to all neighbors

17
Discussion Link-State Routing

• Plusses
• Simple, determines the optimal route most of the
  time
• Used by OSPF
• Minuses
• Might have oscillations
• Avoid using load as cost metric, reduce herding
  effect

1
1e
0
2e
0
0
0
0
e
0
1
1e
1
1
e
recompute
recompute Least loaded gt Most loaded
Initially start with almost equal routes
everyone goes with least loaded
18
Is our routing algo scalable?

• Route table size grows with size of network
• Because our address structure is flat!
• Solution have a hierarchical structure
• Used by OSPF
• Divide the network into areas, each area has
  unique number
• Nodes carry their area number in the address 1.A,
  2.B, etc.
• Nodes know complete topology in their area
• Area border routers (ABR) know how to get to any
  other area

19
Hierarchical Addressing
Zone 2
0
1
S1
1
0
2
S2
2
3
1
0
2
Zone 3
20
IP has 2-layer addressing

• Each IP address is 32 bits
• Network part which network the host is on?
• Host part identifies the host.
• All hosts on same network have the same network
  part
• 3 classes of addresses A, B and C

18.26.0.1
host
network
32-bits
1 0 net host
110 net host
2 14 16 bits
3 21 8 bits
21
IP addressing

• The different classes
• Problems inefficient, address space exhaustion

class
1.0.0.0 to 127.255.255.255
A
network
0
host
128.0.0.0 to 191.255.255.255
Unicast
B
192.0.0.0 to 223.255.255.255
C
224.0.0.0 to 239.255.255.255
D
Multicast
240.0.0.0 to 255.255.255.255
reserved
E
Reserved
1111
22
IP addressing CIDR

• Classless InterDomain Routing
• network portion of address of arbitrary length
• address format a.b.c.d/x, where x is bits in
  network portion
• Examples
• Class A /8
• Class B /16
• Class C /24

23
Internet Protocol Datagram
IP protocol version Number
32 bits
total datagram length (bytes)
type of service
head. len
header length
ver
length
for fragmentation/ reassembly
fragment offset
type of data
flgs
16-bit identifier
max number remaining hops (decremented at each
router)
upper layer
time to live
Internet checksum
32 bit source IP address
32 bit destination IP address
upper layer protocol to deliver payload to
E.g. timestamp, record route taken, pecify list
of routers to visit.
Options (if any)
data (variable length, typically a TCP or UDP
segment)
24
Datagram Portability

• IP Goal To create one logical network from
  multiple physical networks
• All intermediate routers should understand IP
• IP header information sufficient to carry the
  packet to destination
• Goal Run over anything!
• Problem
• Physical networks have different MTUs
• max. transmission unit 1500 for Ethernet, 48
  for ATM
• Solution 1
• Fit everything in the MTU (!)

25
IP Fragmentation Reassembly

• Solution 2 (the one used)
• If packet size gt MTU of network, then fragment
  into pieces
• Each fragment is less than MTU size
• Each has IP headers frag bit set frag id
  offset
• Packets may get refragmented on the way to
  destination
• Reassembly only done at the destination
• What is a good initial packet size?

reassembly
fragmentation in one large datagram out 3
smaller datagrams